1. Field of the Invention
The present invention generally relates to an optical pick-up device, and more particularly to an optical pick-up device in which data is recorded and/or reproduced and/or deleted on a recording medium.
2. Description of the Related Art
FIG. 1 shows a conventional optical-pick-up device. In the optical-pick-up device, a light emitted from a semiconductor laser (LD) 1 as a laser light source is parallelized by a collimating lens 2. After the parallel light passes a beam splitter 3, the light is deflected upwardly by a deflection prism 4. After the deflected light passes through a quarter-wavelength plate 5, the light is focused on a recording surface of an optical disc 7 as an optical data recording medium by an objective lens 6. A reflected light A from the optical disc 7 is parallelized by the objective lens 6 and changed in a deflection direction by the deflective prism 4. After the light passes the deflection prism 4, the light is reflected by the beam splitter 3 and introduced to a signal detection optical system 8. The reflected light A from the optical disc 7, which is introduced in the signal detection optical system 8, is focused by a detection lens 9. A part of the light passing a beam splitter 10 is introduced to a light receiving element 11 and reflected to a light receiving element 12. The light receiving element 11 is divided into six light receiving faces a-f and the light receiving element 12 is divided into three light receiving faces g, h, i. In the above device, a data signal Rf, which is a signal recorded on a recording face, a focus error signal Fe (beam-size method) and a track error signal Te are obtained by the following formulae. EQU Rf=a+b+c+d+e+f+g+h+i EQU Fe=(a+d+c+f+h)-(b+e+g+i) EQU Te=(a+b+c)-(d+e+f)
FIGS. 2A and 2B show conventional signal detection optical systems 8. In a signal detection optical systems 8 shown in FIG. 2A, the reflected light A focused by the detection lens 9 is given an astigmatism by an astigmatism lens 13 and introduced to a light receiving element 14. The light receiving element 14 is divided into four light receiving faces a-d. In the above system, a data signal Rf, a focus error signal Fe and a track error signal Te are obtained by the following formulae. EQU Rf=a+b+c+d EQU Fe=(a+c)-(b+d) EQU Te=(a+d)-(b+c)
The formulae are determined by an astigmatism method.
In the signal detecting optical system 8 shown in FIG. 2B, the reflected light A focused by the detected dens 9 is filtered by the knife edge prism 15. An unfiltered light is introduced to the light receiving element 16 and a filtered light is introduced to the light receiving element 17. The light receiving element 16 is divided into two light receiving faces a, b and the light receiving element 17 is divided into two light receiving faces c, d. The data signal Rf, the focus error signal Fe and the track error signal Te are determined as follows. EQU Rf=a+b+c+d or Rf=c+d EQU Fe=a-b EQU Te=c-d
The above formula is determined by a knife edge method.
Hereinafter, the deflection on the recording face of the optical disc 7 is described, referring to FIGS. 3 and 4. A light spot P focused by the objective lens 6 is diffracted by pit lines of the optical disc 7 of a regular CD. FIG. 4A shows a ROM disc face on which recesses and projections are formed in advance. In this case, the recess is a pit portion and the projection is a non-pit portion. A light reflected by the pit portion and a light reflected by the non-pit portion have different phase, which leads to an interference therebetween. The phase difference .delta. therebetween is defined by the following formula, in which h represents a pit height, n represents a reflective ratio of the base and .lambda. represents a wavelength of the laser light. EQU .delta.=2.pi.nh/.lambda.
When the data is recorded based on the phase change as shown in FIG. 4B, a light reflected by the mark portion 22a and a light reflected by the non-mark portion 22b, each of which has different phases, generates the interference therebetween.
When a reflected light A, which is diffracted by the pit lines of the optical disc 7, returns to the objective lens 6 having a numerical aperture NA, the deflected light of a zero-order light 20a and a first-order light 20b is absorbed. After that, the light is introduced to the light receiving elements 11, 12 of the optical system shown in FIG. 2. A light amount in an overlapping area in which the zero-order light 20a and the first-order light 20b interfere is changed to detect the pit. In this case, the pit is detected as the data signal. The pit in this context includes the phase pit due to the recesses and projections of the recording face and the dot difference in the reflective index from the mirror face. When the zero-order light 20a and the first-order light 20b is introduced to light receiving elements 11, 12 according to the mechanism described above, the data signal Rf can be determined as a total light amount, according to the above formula.
When the pit is smaller, the diffraction angle .theta. becomes larger. When the pit is smallest, the overlapping area of the zero-order light 19a and the first-order light 19b becomes smaller. In this case, the pit is difficult to be detected. That is, a change in a short pit signal appears at a periphery of a far field pattern (FFP) of the detected light when the pit gets smaller and highly-condensed, instead a change in a long pit signal appears at a central portion of the detected light FFP since the deflected angle .theta. is small.
In the conventional optical pick-up device shown in FIG. 1, the data signal Rf (Cf. the above formula) recorded in the optical disc 7 is determined based on the total light amount of the overlapping area of the zero-order light 20a and the first-order light 20b, in which the light amount is changed due to the pit shape, and the zero-order light 20a, which includes a relatively large amount of noise. However, only by detecting a change in the total amount of light, when the recording medium has the small pits and a high density, the overlapping area becomes small compared to the area of the zero-order light 20a, and a ratio of the noise element becomes larger. Thus, the S/N ratio thereof is lowered.
In the CD as the recording medium, the shortest signal corresponding to the shortest pit is a 3T signal, and the longest signal corresponding to the longest pit is an 11T signal, in which T represents the fundamental clock period. In this case, in order to read the pit recorded at a high density with a high S/N ratio, the resolving power must be improved by increasing the amplitude of the 3T signal or the S/N ratio must be improved by reducing the noise of the 3T signal. In order to improve the amplitude detected from the shortest pit, a spot radius of the light spot P for reading is reduced to improve the resolving power of the spot. It is an indispensable project to provide a LD handling a shorter wave and an objective lens having a higher NA, which cannot be accomplished soon. In the method in which the noise in the 3T is reduced, the zero-order light 20a to be a noise in the reflected light A is reduced. However, in the conventional signal detecting method, a data signal Rf is determined by a variation of the total amount of received light. Therefore, when data is recorded at a high density in the CD, the noise can not be reduced. Thus, the S/N ratio is lowered and the signal cannot be detected accurately.
Recently, in order to provide a high-density recording and reproduction, a new technique called super-dissolution has been developed and an optical pick-up device having a higher dissolving power than the conventional one shown in FIG. 1 has been proposed (Optics, Vol. 21, No. 5, 1992/5, Page 342-345, Journal of Television Society, Vol. 48, No. 5, P 557-560). In the device using the super-dissolution, the small light spot over a deflection limit can be provided by changing a light intensity and a phase of the central portion of the beam before focussing. Since the light spot forms a relatively strong side lobe, the side lobe element included in the reflected light is shaded by a slit when the signal is reproduced. Only the separated main lobe element is received to obtain the reproduction signal. However, in the device using the super-dissolution for a high density record, light utilization efficiency is lowered by the slit for shading. A spot diameter of the slit is less than 100 .mu.m and accuracy in positioning of the slit is not realized when the device is made small.
Also, in signal reproduction methods, a new medium (super-dissolution optical magnetic disc) can be used (Applied Physics, Vol. 61, No. 3, 1992, Page 250-253). However, it is still in the research stage and is not practical.